The pks for Histidine are 1.8, 6.0, and 9.3. Of 100 molecules of Histidines at pH 6.0, how many are net positively charged? ● What is the ratio of uncharged to charged side chains at pH 7.0? Calculate the pl value of histidine.

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Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
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## A Problem to Solve

**The pKs for Histidine are 1.8, 6.0, and 9.3.**

### Questions:
1. **Of 100 molecules of Histidines at pH 6.0, how many are net positively charged?**
2. **What is the ratio of uncharged to charged side chains at pH 7.0?**
3. **Calculate the pI value of histidine.**

### Diagram Explanation:

Below the questions, there is a detailed diagram showing the ionization states of histidine at different pH levels.

- **At pH < 1.8**: The structure of histidine shows:
  - Amino group (\(NH_3^+\)) with a positive charge.
  - Carboxyl group (\(COOH\)).
  - Imidazole ring with a protonated nitrogen (\(NH^+\)), also having a positive charge.

  - **Net charge = +2**

- **At pH = 6.0**: The structure of histidine shows:
  - Amino group (\(NH_3^+\)) with a positive charge.
  - Carboxyl group (\(COO^-\)) with a negative charge due to the loss of \(H^+\).
  - Imidazole ring with a protonated nitrogen (\(NH^+\)), also having a positive charge.

  - **Net charge = +1**

- **At pH = 9.3**: The structure of histidine shows:
  - Amino group (\(NH_2\)) without a charge due to the loss of \(H^+\).
  - Carboxyl group (\(COO^-\)) with a negative charge.
  - Imidazole ring without a protonated nitrogen (\(N\)) without a charge.

  - **Net charge = 0**

- **At pH > 9.3**: The structure of histidine shows:
  - Amino group (\(NH_2\)) without a charge.
  - Carboxyl group (\(COO^-\)) with a negative charge.
  - Imidazole ring with a nitrogen (\(NH\)), without a charge.

  - **Net charge = -1**

The graph is a reference guide showing the protonation states and net charges of histidine at different pH levels, which aids in answering the
Transcribed Image Text:## A Problem to Solve **The pKs for Histidine are 1.8, 6.0, and 9.3.** ### Questions: 1. **Of 100 molecules of Histidines at pH 6.0, how many are net positively charged?** 2. **What is the ratio of uncharged to charged side chains at pH 7.0?** 3. **Calculate the pI value of histidine.** ### Diagram Explanation: Below the questions, there is a detailed diagram showing the ionization states of histidine at different pH levels. - **At pH < 1.8**: The structure of histidine shows: - Amino group (\(NH_3^+\)) with a positive charge. - Carboxyl group (\(COOH\)). - Imidazole ring with a protonated nitrogen (\(NH^+\)), also having a positive charge. - **Net charge = +2** - **At pH = 6.0**: The structure of histidine shows: - Amino group (\(NH_3^+\)) with a positive charge. - Carboxyl group (\(COO^-\)) with a negative charge due to the loss of \(H^+\). - Imidazole ring with a protonated nitrogen (\(NH^+\)), also having a positive charge. - **Net charge = +1** - **At pH = 9.3**: The structure of histidine shows: - Amino group (\(NH_2\)) without a charge due to the loss of \(H^+\). - Carboxyl group (\(COO^-\)) with a negative charge. - Imidazole ring without a protonated nitrogen (\(N\)) without a charge. - **Net charge = 0** - **At pH > 9.3**: The structure of histidine shows: - Amino group (\(NH_2\)) without a charge. - Carboxyl group (\(COO^-\)) with a negative charge. - Imidazole ring with a nitrogen (\(NH\)), without a charge. - **Net charge = -1** The graph is a reference guide showing the protonation states and net charges of histidine at different pH levels, which aids in answering the
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